Hydrogen storage is a technology critical to a wide variety of applications, some of the most prevalent being fuel cells, portable power generation, and hydrogen combustion engines. Such applications would benefit substantially from hydrogen storage alloys capable of absorbing and desorbing higher amounts of hydrogen as compared to present day commercially available hydrogen storage alloys. Hydrogen storage alloys having the hydrogen absorption and desorption characteristics of the present invention will benefit such applications by providing longer operating life and/or range on a single charge for hydrogen power generators, fuel cells, and hydrogen internal combustion engines.
In the past, considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are being rapidly depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal, natural gas and other hydrocarbons, or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Conventionally, hydrogen has been stored in a pressure-resistant vessel under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas would weigh about thirty times the weight of a container of gasoline. Additionally, transfer of a large sized vessel is very difficult. Furthermore, storage as a liquid presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below −253° C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Certain metals and alloys allow for the reversible storage of hydrogen. In this regard, they have been considered as a possible form of hydrogen-storage, due to their large hydrogen-storage capacity. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase storage of hydrogen in a metal or alloy system works by absorbing hydrogen through the formation of a metal hydride under a specific temperature/pressure or electrochemical conditions, and releasing hydrogen by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time, since they are formed by the insertion of hydrogen atoms into the crystal lattice of a metal. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature/pressure, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas, and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
The hydrogen storage capacity per unit weight of material is an important consideration in many applications, particularly where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of a hydrogen fueled vehicle making the use of such materials. A low desorption temperature is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, fuel cells, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
Hydrogen storage materials include a variety of metallic materials such as Mg, Mg—Ni, Mg—Cu, Ti—Fe, Ti—Mn, Ti—Ni, Mm—Ni and Mm—Co alloy systems (wherein, Mm is Misch metal, which is a rare-earth metal or combination/alloy of rare-earth metals). However, basic hydrogen storage metals or alloys do not have all of the properties required for a storage medium with widespread commercial utilization.
Of these materials, the Mg alloy systems can store relatively large amounts of hydrogen per unit weight of the storage material. However, heat is needed to release the hydrogen stored in the alloy, because of its low hydrogen dissociation equilibrium pressure at room temperature.
The use of rare-earth (Misch metal) alloys have several drawbacks. Although they can be formulated to absorb and release hydrogen at room temperature, based on the fact that they have a hydrogen dissociation equilibrium pressure on the order of several atmospheres at room temperature, their hydrogen-storage capacity per unit weight is lower than desirable.
Transition metal alloy systems, like the Ti—Fe alloy system, have the advantage that they can be formulated with a hydrogen dissociation equilibrium pressure of several atmospheres at room temperature. However, these alloys typically require a high temperature of about 350° C. and a high pressure of over 30 atmospheres for initial hydrogenation, while providing only relatively low hydrogen absorption/desorption rates. Also, these alloy systems exhibit a hysteresis problem which hinders the complete release of hydrogen stored therein.
Under the circumstances, a variety of approaches have been made to solve the problems of the prior art and to develop an improved material which has a high hydrogen-storage efficiency, a proper hydrogen dissociation equilibrium pressure and a high absorption/desorption rate.